1. Field of the Invention
This invention relates to viscosity stabilization in water-based hydraulic fluids thickened with water-soluble polymers.
2. Prior Art
In the technology of hydraulic power transmission, mechanical power is imparted to a fluid called "a hydraulic fluid" in the form of pressure by means of a hydraulic pump. Power is utilized where desired by tapping a source of said hydraulic fluid thus transforming the power as pressure back to mechanical motion by a mechanism called a hydraulic motor. The hydraulic fluid is utilized as a pressure and volume transmitting medium. Any non-compressible fluid can perform this function. Water is the oldest fluid used for this purpose and is still sometimes used alone for this purpose. In the prior art, there has been a heavy emphasis on the development of petroleum oils for use as hydraulic fluids and, consequently, much of the equipment utilized with hydraulic fluids has been designed and manufactured specifically for use with petroleum oils. A petroleum oil in comparison with water as a hydraulic fluid possesses the advantage of inhibiting the development of rust of the ferrous components of the mechanical equipment utilized in conjunction with hydraulic fluids, (i.e., hydraulic pumps, motors, etc.) and in preventing wear of the machinery since the hydraulic fluid must lubricate the equipment. Petroleum oils have a second advantage over the use of water as a hydraulic fluid in that the petroleum oils normally exhibit a substantially higher viscosity than water and thus contribute to reduction of the leakage of the fluid in the mechanical equipment utilized. In addition, the technology relating to additives for petroleum oils has developed to such an extent that the viscosity, foam stability, wear prevention and corrosion prevention properties of such petroleum oil-based hydraulic fluids can be further enhanced by the use of said additives.
Over the past 25 years, various substitutes for petroleum oil-based hydraulic fluids have been developed in order to overcome one of the major deficiencies of petroleum oils, namely, flammability. Recent interest in the use of hydraulic fluids having up to 99 percent or more of water has resulted from the higher cost of petroleum oils and recent emphasis on problems of ecologically suitable disposal of contaminated or spent petroleum oil-based hydraulic fluids.
Metalworking fluids of the so-called "soluble oil" type have been considered for use as hydraulic fluids. Such fluids contain mineral oil and emulsifiers as well as various additives to increase corrosion resistance and improve antiwear and defoaming properties. Such fluids, when used as hydraulic fluids, are not generally suitable for use in ordinary industrial equipment designed specifically for use with the petroleum oil-based hydraulic fluids since such fluids do not adequately prevent wear damage in some types of pumps and valves of such equipment. However, such fluids have found application in specially designed, high cost, large size equipment which, because of said large size and thus inflexibility, is not suitable for use in most industrial plants. The soluble oil hydraulic fluid usage has thus been quite limited; usage has been largely confined to large installations where flexibility and size are not critical, such as in steel mills.
It is also known to use, in equipment designed for use in mineral oil-based hydraulic fluids, flame-resistant glycol-water based hydraulic fluids such as are disclosed in U.S. Pat. No. 2,947,699. Up until recently, water-based hydraulic fluids containing about 70 to 99+percent water, have had very poor lubricating characteristics. While hydraulic fluids are used primarily to transmit forces, it is necessary that they provide lubrication for the impeller, rings, vanes, gears, pistons and cylinders and other mechanical parts of hydraulic pumps in such systems in order to prevent excessive wear on such parts.
Many prior art fluids, such as the petroleum oil type, are highly flammable and unsuitable for certain uses where such fluids have frequently been the source of fire. Where these fluids are used to control such industrial operations as heavy casting machines, which are operated largely by hydraulic means, danger of fire exists. Therefore, there is a growing demand for hydraulic fluids characterized by reduced flammability.
Hydraulic fluid compositions having water as a base are disclosed in U.S. Pat. Nos. 4,151,099 and 4,138,346. These patents disclose fluids comprising 1) a sulfur containing compound and 2) a phosphate ester salt. The U.S. 4,151,099 patent also includes a water-soluble polyoxyethylated ester of an aliphatic acid and a monohydric or polyhydric aliphatic alcohol, either one or both said acid and said alcohol being polyoxyethylated. These hydraulic fluids are optionally thickened with a polyglycol thickener.
In U.S. Pat. No. 2,558,030 a hydraulic fluid is disclosed having an organic polymeric thickener such as copolymers of ethylene oxide and propylene oxide having high molecular weight which includes ethylene glycol as well as sodium mercaptobenzothiazole. However, there is no disclosure of dimercaptothiadiazole.
U.S. Pat. Nos. 3,909,448 and 2,803,140 disclose 2,5-dimercaptothiadiazole as an antioxidant for organic materials. However, they do not relate to hydraulic fluids.
U.S. Pat. No. 2,602,780 discloses hydraulic fluids with soluble polymeric thickeners such as copolymers of ethylene oxide and propylene oxide having molecular weights of 15,000 to 20,000. In combination therewith it discloses the use of ethylene glycol and propylene glycol. However, the ethylene glycol and propylene glycol are employed as freezing point depressants and are included in a list of many freezing point depressants. The patent also discloses the inclusion of thiazoles and substituted thiazole salts such as alkali metal mercatobenzothiazoles.
U.S. Pat. No. 2,751,356 discloses a hydraulic pressure transmitting fluid comprising polymerized lower alkylene glycols containing as additives ethylene glycol and propylene glycol and also the sodium salt of mercaptobenzothiazole. These latter products are provided in a list of diluents or solvents.
From the above it can be seen that it is well known in the art to employ organic polymeric thickeners such as copolymers of ethylene oxide and propylene oxide having high molecular weight. However, such thickening agents, particularly when heated, are subjected to oxidizing conditions which result in the loss of the thickening effect, i.e. a loss of viscosity in the hydraulic fluid.